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Warranty Strong: How RH Testing Limits Flooring Failure

Recurrent moisture content is a plague on the modern flooring industry, especially as it applies to shrinking building schedules and budgets. When builders apply flooring over improperly dried concrete slabs, high moisture content levels can trigger perpetual problems that can jeopardize a floor, including adhesive breakdown, debonding, osmotic blisters, staining, efflorescence, curling or expansion.

In recent years, the flooring products industry’s shift away from volatile organic compounds (VOCs) has created a new generation of adhesives that are more sensitive to concrete moisture content than ever. Most manufacturers provide product specifications for flooring professionals, but the issue is one of concrete slab moisture measurement.

If the end result is to be a strong and resilient floor, the quest necessitates precise and thorough concrete moisture assessment.

The Influences of Process

When industry warranties and reputation rest on integral flooring installation, builders refer to industry guidelines. The American Society for Testing and Materials (ASTM) has two such guidelines for moisture assessment of concrete slabs: ASTM F1869-11 and ASTM F2170-11. Each guideline refers to a specific concrete slab testing methodology. However, it is important to first understand the environmental factors that influence fluctuations in concrete moisture content levels.

Concrete Moisture 101

Moisture is present from the moment that a concrete mixture is conceived and created. Once a slab is poured, moisture dissipates through surface evaporation as the slab cures and dries. As that moisture is drawn to the slab surface, moisture content inside the slab redistributes for the same purpose: to rise and evaporate. Thus, installers must allow sufficient time for moisture to do what it does through this natural redistribution and evaporation process. Under ideal conditions, the “rule of thumb” is to allow approximately 30 days of slab drying time for each inch of concrete slab thickness.

When was the last time the building world was ideal? Stakeholders must adapt to a consistent and universal truth: It is vital to manage moisture content based on a working understanding of why it fluctuates. This may seem daunting, but the answer is simpler than many people expect.

Slab moisture levels are influenced by the original concrete mixture, but also by the slab’s surrounding environment. Concrete slab moisture levels can change due to poor drainage from beneath the slab, capillary action (known as wicking) from sub-surface moisture, or from moisture-rich additives applied to the concrete surface. Although these influences may be natural to some degree, they invariably influence concrete slab drying times.

Ambient relative humidity (RH) and temperature are powerful variables. The RH of ambient air can cause a slab’s surface evaporation rate to increase or decrease. When RH is high, concrete absorbs water vapor from the air; when RH is low, concrete emits water vapor into the air. Changes in ambient RH influence slab moisture content even before floor coverings are applied. Once covered, slab moisture continues to migrate from the bottom of the slab until moisture content is proportionately equilibrated. Even if initial surface moisture content was low, after the floor covering application RH will continue to rise from the bottom of the slab. In fact, moisture levels do not fully settle until after floor coverings are installed.

Of course, the most proactive approach is constructed by design. Builders have numerous ways to ensure that concrete moisture, a natural presence in the concrete flooring process, has avenues by which it can exit a concrete slab.

Make sure drainage runs away from a newly-poured slab and that the grade is adequate for local conditions.

Specify a true vapor retarder, with a minimum thickness of 10 mils, under the slab to prevent moisture seeping in through the ground.

the water-to-cement ratio in concrete mixtures. Various admixtures suffice in minimizing initial moisture content while ensuring a workable mix.

Allow the slab to dry naturally at service conditions. Protect it from the elements, and avoid maintenance or cleaning processes that will wet the slab.

The Trouble with Reactive Testing

For decades, builders and flooring installers have assessed concrete moisture using the anhydrous calcium chloride (CaCl) test of moisture vapor emissions. The moisture vapor emission rate (MVER) is the amount of moisture emitted into a salt compound sealed to the surface of concrete with an enclosed moisture dome. The anhydrous CaCl test assesses slab surface moisture content for 60-72 hours. The resulting MVER had been deemed as a reliable method for determining concrete slab readiness.

However, the 21st century brought renewed inquiry regarding the anhydrous CaCl test method. The construction industry began to recognize that slab performance, as assessed by CaCl, was often less stable than the MVER first indicated. Specifically, professionals focused on the fact that by nature, the anhydrous CaCl test assessed moisture conditions only at the surface of the slab. It was time to re-evaluate the historic tool of concrete moisture assessment: the anhydrous CaCl test and its accompanying industry standard, ASTM F1869.

How could a general contractor and specifier help the flooring installer prevent future moisture problems?

RH Measurement Success

RH testing has now been acknowledged as a legitimate means of assessing overall slab moisture content levels. In fact, the ASTM International approved ASTM F2170-11 as an alternative to F1869. Ultimately, the industry has recognized that RH is a legitimate and accurate means of determining concrete slab readiness.

How? Through extensive research begun in Sweden in the 1990s, and through further ongoing testing, it has been shown that sensors placed at 40% of the concrete slab depth (for slabs drying from one side) ably predict final moisture conditions if the slab were to be sealed at that point in the drying process. There is no need to probe any further to obtain precise, reliable RH measurement with the Rapid RH®.

The Benefits of Proactive Testing

The Rapid RH® is available as either a single-use or reusable system and provides a state-of-the-art, cost-effective, and efficient way to accurately assess the moisture condition of the slab. The Rapid RH® sensors equilibrate, in most cases within 60 minutes, whereas other RH measurement systems can take many more hours. While ASTM compliance requires 72 hours for recording the final reading, the speed and accuracy of the Rapid RH® test method allows flooring installers to make business-critical, non-installation decisions in as little as one hour in most cases.

In addition, subsequent readings couldn’t be simpler. By keeping the Rapid RH® Smart Sensor installed in the hole, one can record additional readings within seconds, without having to wait for re-equilibration at each test hole.

Most important, use of the Rapid RH® concrete moisture measurement system helps to limit contractor liability, by ensuring acceptable moisture conditions and by providing full ASTM F2170 compliance.

That’s great insurance and certainly contributes to optimal results for each of your flooring projects.

Jason has 17 years' experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH concrete moisture tests. He currently works with Wagner Meters our Rapid RH product sales manager.